Neurodegenerative diseases typified by the intracellular accumulation of the microtubule-associated protein tau, collectively known as tauopathies, are both clinically and pathologically devastating. The neurofibrillary tangles formed by the abnormal hyperphosphorylation and deposition of tau suggests a tau- mediated mechanism of disease and, therefore, is a primary target of interest for the development of effective therapies. Interestingly, select tauopathies present with an imbalance of tau isoform deposition, exhibiting either predominant 4R or 3R tau inclusions. The overall hypothesis of this proposal is that a greater 4R:3R tau ratio induces tau pathogenesis, and splicing alterations to reduce the 4R:3R ratio will mitigate neurodegenerative deficits. We plan to use antisense oligonucleotides (ASOs) to manipulate the splicing patterns of tau to promote or prevent a diseased state in mouse models and subsequently performs a comprehensive evaluation of tau isoform expression in human tauopathy to help direct ASO-treatment strategies. Our results will be essential in implicating a greater 4R:3R tau ratio as pathogenic and identifying a novel target for the treatment of tauopathies. Aim 1 will use ASOs that decrease the 4R:3R ratio in a mouse model of FTDP-17 characterized by elevated 4R expression. Preliminary studies have shown amelioration of behavioral deficits and tau pathology when ASO treatment was initiated prior to the onset of pathology. In the proposed experiments, ASOs will be administered to aged mice to demonstrate reduced tau pathology and functional improvement following disease progression. This result will help support a 4R tau-directed therapy for 4R tauopathies. To specifically implicate a greater 4R:3R ratio as pathogenic, ASOs that drive 4R expression without changes to total tau will be used in a human tau-expressing mouse model (Aim 2). The expectation is that an increased 4R:3R ratio will induce greater hyperphosphorylated tau and biochemical tau pathology. Subsequently, Aim 3 will comprehensively examine the RNA and protein composition of 3R and 4R tau isoform expression in human tauopathies. We will analyze postmortem human tissues from tauopathies and healthy controls to investigate whether isoform-specific pathology is mediated by transcriptional or translational changes in tau isoforms. This information will help define the nature and future directions of ASO therapies for tauopathies. Upon completion of these aims, we will have determined the influence of 4R:3R tau ratio on age-related tau pathology and cognition in models of human neurodegeneration. These studies also may lend critical support for ASOs as a promising, novel therapy for 4R tauopathies.